Modern cellular telecommunication systems comprise complicated structures of network elements. New developments, such as macro diversity combining, increase the complexity of a network. Macro diversity combining (MDC) refers to a situation, where terminal unit of a cellular telecommunication system has simultaneous connections to at least two base stations, in which case any given data unit can be routed to the terminal unit or from the terminal unit via at least two routes. The set of such base stations is called the active set. Macro diversity combining can be utilized especially in spread spectrum technology based systems, when the terminal unit is near the border of cells or in an area, where more than one cells overlap at least partly. One of the advantages of macro diversity combining (MDC) is the resistance to various disturbances occurring in the propagation paths. Since fading and disturbances occurring in a given propagation path can be compensated using data transmitted via another propagation path, MDC provides a better quality of transmission than schemes based on use of single connections.
In cellular systems based on spread spectrum technology, it is advantageous to minimize the transmission power of mobile and base stations in order to maximize the capacity of the system. Macro diversity combining allows the use of lower transmitting power than in a system, where all other things being equal, the radio link between the mobile station and the network is carried by a single connection. On the other hand, spread spectrum technology offers good possibilities to combine signal components arriving to the combination location with varying delays and power levels, due to different propagation paths or macro diversity paths. Due to these reasons, the use of macro diversity combining will increase in the future. The most common application of spread spectrum technology is the CDMA (Code Division Multiple Access) cellular telecommunication technology.
An example of a radio network configuration providing macro diversity combining is shown in FIG. 1. Downlink data is transmitted from the protocol control block 32 in the first radio network controller (RNC) RNC130 to the first splitting unit 34 in the RNC1, which splitting unit replicates the downlink data stream into one stream towards a second splitting unit 34 in the RNC1, and another data stream towards the splitting unit 34 in RNC2. The second splitting unit 34 in RNC1 replicates the received data stream into one stream towards a first base station (BS) 20 and another data stream towards a second base station 20. The splitting unit 34 in RNC2 further replicates the received data stream into one stream towards a third base station 20 and another data stream towards a fourth base station 20.
Uplink data from the mobile station (MS) 10 is received separately by each base station 20. The first and second base stations send the received data packets to the second combining unit 33 in RNC1, and the third and fourth base stations send the received data packets to the combining unit 33 in RNC2. When the combining units 33 have received the packets, they combine the packets and send only one packet further. The first combining unit 33 receives the output from the other two combining units, and combines the data packets, and forwards the combined data packets to the protocol control block 32.
The combining units 33 may perform the combining in many ways. For example, the units may select only one of the two received packets and send the selected packet. They also may combine the signals of the two packets and send the combined packet.
For clarity, the splitting 34 and combining 33 units are represented by a single symbol in FIG. 1.
In such a configuration, problems are caused by the fact that each downlink packet has to be sent from each BS towards the MS at roughly the same time or within a small time window, while the delay from the RNC to base stations varies. The delay variation may be caused by numerous reasons. For example, physical distance and transmission links between base stations and the radio network controller changes when the base station (BS) or base stations used by a mobile station (MS) is/are changed. Further, each part of the chain of transmission links can have different properties such as bitrate and characteristic delay variation, which properties may change due to variations in traffic or for other reasons. Delays are in turn caused for example by the physical length of the transmission links, and processing of the transmitted data in the network entities. Such processing can for example be encoding, splitting, or combining of data packets. A further problem in downlink direction is, how to keep the difference between the sending time of packets from RNC and transmission time of the packets by the base stations in minimum, while still fulfilling the edge condition, that each BS receives the data before it has to be transmitted.
In the uplink direction, a problem is how to determine the combining time for each combining/splitting unit, while still fulfilling the edge condition that the combined data packet is to be received by the protocol control block by the defined time. The combining time is the point in time, by which a combining unit must send the combined data to its output, regardless of whether all packets to be combined are received or not.
An additional problem, which is not solved by the prior art structures, is how to take advantage of statistical multiplexing gains resulting of multiplexing of transmission links, since the more efficiently the data is multiplexed, the longer the average delay is and especially, the longer the delay variation is. Statistical multiplexing gains refer to savings in the use of data transmission resources obtained, when the data packet transmission times and other transmission parameters of bearers having relatively loose delay requirements are adjusted in order to accommodate data packets of bearers having stricter delay requirements.
The prior art solutions do not address all of the previous problems. One prior art solution is used in the GSM system, where in the downlink direction, a base station can indicate to a transcoder unit, that the transcoder unit has to advance the transmission times of frames sent towards the base station. This mechanism is explained in detail in the specification GSM 08.60.